Rainbows are circular and form at 40-42 degrees angle. Bonus: double rainbows

SciShow explains how do rainbows form: you need water droplets in the air, the sun low in the sky behind you and you, of course. When the sun is behind you in the morning or evening, when it is closer to the horizon, you will be able to see a rainbow if there are some water droplets floating in the air.

Looking at them at 40 – 42 degrees you will be able to see the half-circled rainbow which is, in fact a full circle. Some pilots or guys that were hiking up in the mountains were able to see full circles rainbows.